Side band reversal transmission system



Nov. 21, 1933. J H HAMMOND, JR 1,935,776

SIDE BAND REVERSAL TRANSMISSION SYSTEM Filc 1 May 18, 1929 I 4Sheets-Sheet s F/L TEE F/L TEE .90

{IMLE fllllilik INVENTOR JOHN HAYS HAMMONQ JR.

' BY 7 7 W ATTORNEY NOV. 21, 1933. J. HAMMQNQJR 1,935,776

SIDE BAND RE'VERSAL TRANSMISSION SYSTEM Fi led May 18, 1929 4Sheets-Sheet 4 INVENTOR JOHN HAYs HAMMOND .m.

- ATTORNEY ?a.tented Nov. 1933 more srArEs TENT SIDE BAND REVERSALTRANSMISSION SYSTEM 10 Claims.

The object of this invention is to provide a system of radiocommunication by which intelligence can be transmitted subject to littleor no interference from other radio signals or from atmosphericconditions.

Another object of this invention is to provide a system of telegraphyand telephony which will produce a small amount of interference withrespect to nearby receivers of the usual type, without, however,preventing strong signals from being received by special types ofreceivers.

It is also an object of this invention to provide a system of telegraphyand telephony which cannot be intelligently received by the ordinarytype of receiver.

Other objects and uses of this invention will appear from the followingdescription taken in connection with the accompanying drawings.

This invention relates to transmission by modulation methods or by awave which may be compounded from a plurality of continuous waves. Moreparticularly, this invention relates to the special feature that thesignalling is accomplished by the reversal or other alteration of thephase of any of the constituents of the composite wave.

Having thus briefly described my invention, attention is invited to theaccompanying drawings in which:

Figs. 1, 2, 3, 4, and 5 are each diagrams indicating the envelope ofdifferent synthetic waves. Fig. 6 is a circuit diagram of a transmitterfor carrying out the objects of my invention.

Fig. '7 is a circuit diagram of a receiver for cooperation with thetransmitter of Fig. 6.

Fig. 8 is a circuit diagram of an alternative form of transmitter forcarrying out the principles of my invention, and

Fig. 9 is a circuit diagram of an alternative form of receiver forcooperation with the transmitter of Fig. 8.

It is well known that a modulated sine wave is in all respectsequivalent to a plurality of waves. In the present art, these waves aretermed the carrier frequency and the side frequencies. A sine modulatedwave of a carrier frequency of 1,000,000 cycles modulated by a frequencyof An additional characteristic of these three constituent waves whenarranged as in amplitude modulation, is shown by representing them in avectorial sense, withone vector for each wave. At any instant of time,the angle between the vector representing the carrier C and the vectorS+ representing the upper side frequency must be equal to the anglebetween same carrier vector C and the vector 3- representing the lowerside frequency. Thus, when the waves C and 8+ are 'at their maximumpositive value, the wave S- must be at its maximum positive value.Consequently a sinusoidally amplitude modulated wave cannot be producedsynthetically from three sinusoidal waves unless the followingconditions are met.

1. The amplitudes of the side waves must be equal and not more than halfthe carrier wave.

2. The difference between the frequencies of the carrier wave and theupper side frequency must be exactly the same as the difference 'betweenthe carrier frequency and the lower side frequency.

3. With two of the waves established, the phase of the third wave mustbe properly adjusted.

The envelope shown in Fig. 1 is that of composite wave made up of threewaves suitably related to produce a sine modulated wave in accordancewith the above rules. On .the other hand, the envelope shown in Fig. 2is that which would be produced, if, for example, the upper side wavewere altered in phase by 180 from that which would be proper inconjunction with the other constituents to produce the sine modulatedform posite form shown in Fig. 1 is an audio modulated form of 1,000cycle modulation, then a non-oscillating detector operated by a voltageof such wave form will produce chiefly a 1,000 cycle tone, whereas ifactuated by a wave as shown in Fig. 2, it will produce substantially a2,000 cycle tone, or if by a wave as shown in Fig. 3, a resultant ofboth 1,000 cycle and 2,000 cycle tones.

From another point of view, if the three constituents are transmittedand impressed upon a detector, three audio signals result. The first isproduced by the beat action between the carrier and the upper sidefrequency. The second is produced by the beat action between the carrierand the lower side frequency. And the third is produced by the beataction between the upper and the lower side frequencies.

In the present case, the resultant 2,000 cycle current is the same,whether the form of envelope transmitted is that of Fig. 1, Fig. 2, orFig. 3, since it arises from the beat action between the two side waves.The net 1,000 cycle current produced,

. however, depends upon how the individual 1,000

cycle detected currents, due to the carrier and the individual sidefrequency constituents are related as to phase. In the case they are inthe phase relationship which would give the envelope as indicated inFig. 1, they give the maximum net value. In the case they are in suchphase relation as to give the form of envelope shown in Fig. 2, they arein phase opposition and give minimum net value, and in the case they arein such phase relation as to give the form of envelope shown in Fig. 3,they are in phase quadrature to give an intermediate value.

The common problem of transmitters constructed in accordance with thisinvention is to produce three or more waves, related in frequency andamplitude and to control the phase of one or more of the constituentwaves. For telephony, it will be understood that the upper and lowerside frequencies are to be replaced by upper and lower side bands, andthat one band can be operated upon as a group to give 180 phase reversalto all of the constituent frequencies of the group.

A common feature of the receiver for cooperation with such atransmitter, however, as distinguished from receivers of the ordinarytypes, is the selective tuning circuits by which the energy of thecarrier and of one side band may be directly or indirectly diverted toone detector, and energy of the carrier and of the other side band maybe diverted to another detector. By these circuits the two detectedcurrents can be separated. The ordinary type tuner, however, with, butone detector, cannot segregate out the constituent waves and theconstituent currents. Thus, if a wave, the envelope of which isrepresented in Fig. 2, were transmitted to a receiver of the ordinarytype, which cannot separate out the three waves, as for example a singlecircuit tuner with a single detector, the net detected 1,000 cycle notewill be zero and the only notes heard will be a relatively small 2,000cycle note, due to beats between the two side frequencies. On the otherhand, with selective receivers, by diverting part of the carrier and theupper side frequency only, to one detector, a 1,000 cycle note will beproduced in the plate circuit of the detector and by diverting a part ofthe carrier and the lower side frequency current, to another detector,another 1,000 cycle current will be produced. These 1,000 cycle currentswill be out of phase, but either current can be used by itself or thetwo detected currents can be combined by phase reversal to give anadditive effect. Thus a wave form, the envelope of which is representedin Fig. 2, can be used in conjunction with special selective receiversto reproduce a good 1,000 cycle signal, whereas it will produce littleor no 1,000 cycle signal in conjunction with the ordinary receiver.

Fig. 4 shows the envelope of the composite wave, composed of half theenergy of the carrier and all of the energy of the lower side frequencyof the composite wave, the envelope of which is shown in Fig. 2, andFig. 5 shows the envelope of the composite wave composed of half theenergyof the carrier and all of the energy of the upper side frequencyof the same wave. It will be noticed that there is no difference in theshape of the envelope produced in these two cases, but that there is adifference of 180 in the phase with respect to the 1,000 cyclemodulation. It will also be noted, that as explained above, theresulting detected currents will be in opposition, in the plate circuitof the individual detectors.

In one method of using the principle of this invention, the differencebetween the carrier frequency and the side frequency will be a highfrequency, say of the order of 20,000 cycles. The transmitterarrangement for producing the composite waves in case the ratio of thecarrier frequency to difference frequency is not too high, for exampleis shown in Fig. 6.

In this arrangement the difference frequency between the carrierfrequency and the side frequencies is produced and determined by thefrequency of the tube 1 and its associated circuits. The frequency ofthe oscillation circuits is determined principally by the values of theinductances 6 and '7 and the condenser 8.

The carrier frequency is produced and its-frequency determined by thethermionic device 4 and its associated circuits, the frequency of theoscillations produced being principally determined by the value of theinductance 9 and the capacity 10. The method of and circuit forproducing these oscillations is well known in the art and need not bedescribed here in detail.

. Tubes 2 and 3 are high frequency amplifiers. The plate power for theseamplifiers is supplied in part by the battery 11 and in part by thevoltage developed across the inductances 6 and '7 of the differencefrequency oscillator circuit.

Since substantially the same current flows through both coils 6 and '7,it is apparent that when the plate voltage supplied to the tube 2 by theinductive coupling of coil 7 is positive the plate voltage supplied totube 3 by inductive coupling of coil 6 is negative. In other words, thetwo tubes are actuated 180 out ofphase in the low frequency cycle.

The grid circuits of the tubes 2 and 3 are actuated in phase by theirparallel coupling to the high frequency circuit of tube 4. Inconsequence high frequency modulated currents appear in the platecircuits of tubes 2 and 3.

The modulation thus caused will be nearly sinusoidal if the properrelation exists between the direct voltage and the alternating lowfrequency voltage. Moreover, the modulations will be out of phase withthe result, which can readily be shown that with a symmetricalarrangement, the carrier currents are in phase but the side frequenciesare out of phase in the two plate circuits. This is indicated by thearrows C, 8+, and S in each plate circuit which show the instantaneousdirections of carrier and side frequency currents.

Accordingly, energy of the carrier and side frequencies may be separatedout by circuits 1'7, 18,

and 19, which are tuned to the frequencies C, S+, and S, respectively,and suitably coupled to the plate circuits of 2 and 3 by couplings 16,12 and 14, and 13 and 15, respectively. The coupling coil 16 issubstantially free of currents of the side frequencies so that a purecarrier frequency current isdeveloped in the tuned circuit 17.Furthermore, due to the phase difference of the side frequencies incoils 12 and 14, a reversed coupling winding used on one side will causethe carrier I frequency to .be substantially balanced out with respectto circuit 18, whereas it will cause the frequency desired to beaccepted. This circuit need be sharp enough only to discriminate againstthe lower side band frequencies. Similarly, the carrier frequency issubstantially balanced out from circuit 19 which would be tuned only todiscriminate against the upper side band.

This process, therefore, provides a means for separating out the threeconstituent frequencies arising by modulation of the carrier by thedifference frequency. The three constituent frequencies are combined inthe grid circuit of the tube 5 by means of the resistance couplings 22,23, 24, and 25 and means are provided by switches 20 and 21' forreversing the phase of either side frequency current for the purposes ofkeying. The tube 5 is a radio frequency power amplifier tube deliveringits output plate power into an antenna 26 or any other appropriateradiating system.

By means of this arrangement, a composite wave form may be radiated fromthe antenna, the envelope of which wave is as shown in either Fig. 1 or2 depending upon the throw of the reversing switches.

The condition of the current in the antenna may be examined for purposesof making adjustment of the coils and condensers, and for monitorypurposes by means of an aperiodic detector circuit including the tube 28and its associated circuits. The tube 28 is preferably of the threeelectrode type and hasits grid connected through the resistance 30 andbattery 29 to the plate, in order to give it very nearly a straight linerectification characteristic. In consequence, when radio voltage isinduced upon the tube circuit'through the coil 27, there will appear inthe coil 31 a direct current and also a current of the frequencydetermined by the envelope of the composite wave being radiated by theantenna 26. This current may be investigated by the tuned circuitcomprising the coil 32, the meter 33, condensers 34 and 35, and theswitch 36. With the switch 36 to the left, the system is tuned to thedifference frequency, but with the switch to the right, the system istuned to twice the difference frequency. When the circuits are adjustedso as to be in the signalling condition, that is when the composite waveemitted would have the envelope as indicated in Fig. 2, the meter willindicate with the switch to the right and not to the left. If there isan indication with the switch 36 in each position, the transmitterrequires adjustments To receive the energy of the transmitted wave form,a receiver may be employed as shown in Fig. 7. This comprises areceiving antenna or other receiving system 37, a radio frequencyamplifier 38, carrying in its plate circuit a pair of coils 39 and 40.In the plate circuit of the amplifier tube will be reproduced the waveform similar to that radiated by the transmitter antenna, say of Fig. 2when in signalling condition. By means of coupled circuit systems 41 and42, associated with detector tube 45, and coupled circuits 43 and 44,associated with detector tube 46, the energy delivered from the platecircuit is divided, with half of the carrier and all of the lower sidefrequency energy diverted to detector 45, and half of the carrier andall of the higher side frequency energy diverted to detector 46. Thewave forms of envelopes across grids of detectors 45 and 46, inconsequence, are

shown in Figs. 4 and 5, respectively.

These modulations being out of phase, the detected currents in the platecircuits will also be out of phase. In consequence, the output detectedcurrents are to be combined by a differential transformer, with windings4'7 and 48 differently related than 49 and 50. In consequence,

the opposingly phased currents in plate circuits of tubes 45 and 46 willproduce an additive effect in the tuned system 48, 50, 51, and 52 whichis tuned to the difference frequency employed. The current of differencefrequency then may be detected by means of a separate heterodyneoscillator 53, and detector 54, and indicated by telephones 55.Amplifiers may be inserted wherever desired for building up thedifference frequency which may be of audio frequency tone. It isunderstood, of course, that in case the difference frequency is of audiofrequency, no second detection will be required.

This arrangement of the receiver will there fore be responsive when themodulated wave emitted from the antenna is as shown in Fig. 2. But whenthe modulated wave is as shown in Fig. 1, the two wave forms in thedetectors 45 and 46 will be similar, and the plate currents will produceno effect in 'the tuned circuit 48, 50, 51, and 52, and consequently nosignals in the receiver telephones.

Stray disturbances, it is readily seen, produce much less effect withthis double arrangement of detectors. Thus modulated signals due toradio telephone transmitters, for example, forcing the tuned circuits,produce substantially identical wave forms in the plate circuits ofdetectors 45 and 46, but due to the reversal of couplings will produceno forced oscillations in succeeding circuits. Also shock disturbances,producing oscillations in the tuned systems 41, 42, 43, and 44, to agreat extent are lessened over what would get through with but onedetector operating.

In another application of this principle, the side frequencies may be soclose to the carrier wave that it is not feasible to separate them outby filtering. For example, it is not considered feasible to separate outby filtering the upper 5 The transmitter this will increase the phase ofmodulation with respect to one side frequency by 90, but de-, crease thephase of modulation with respect to the other side frequency by 90,giving a total phase displacement by the two channels of 180.

A transmitter for voice modulation in accord- .ance with this plan maybe constructed as indicated in Fig. 8. In this circuit, tube 56 and itsassociated circuits, particularly the circuit 5'1 produce and determinethe frequency of the carrier frequency oscillations. The output of thistube is capacitively coupled by the condenser 58 to a phase shiftingcircuit, comprising resistance 59 and coil 60 in series, paralleled byresistance 61, condenser 62 and coil 63 in series. Constants of theseimpedances are so chosen that the phase of the current in coil 60differs from that in coil 63 by 90. Coils 60 and 63 are the primaries oftwo transformers, the secondaries of which are coils 64 and 65respectively. The coil 64 supplies power for exciting the grid circuitof tubes 66 and 67, which in conjunction with other apparatus producesthe radiated side band energy, and coil 65 supplies power to radioamplifier tube 68 which in conjunction with other apparatus amplifiesand radiates the carrier frequency energy.

Tubes 69 and 70 are push-pull modulators of the plate modulation type,arranged in a balanced manner, with both grids actuated from a commonspeech or other audio frequency source, as indicated, by the microphonecircuit, 71, 72. The grid of the tube 69 becomes most positive when thegrid of tube '70 becomes most negative, and consequently the speech orother audio power is maximum for tube 66 when it is minimum for tube6'7. The output circuits of these tubes have opposite couplings, 73 and'74, to the common output circuit. 1

In consequence, since the grids of tubes 66 and 67 are in phase, thecarrier currents in the plate circuit are in phase and therefore produceno current in circuit 75, 76. But because of the opposition of phase ofplate voltage impressed from modulators 69 and 70 upon radio amplifiers66 and 67, the side band currents in the plate circuit of 66 are inphase opposition to the side band currents in the plate circuit of 67.On account of the opposite couplings from the plate circuits upon thecircuit 76, the side frequency energy from both circuits are additive intheir effects on circuit 75 and '76. Therefore circuit 75-76, tuned to afrequency intermediate between the two side bands, has currents of only.the side band frequencies and no carrier frequency current.

On the other hand, voltage from coil 65 impressed upon amplifier'tube68, produces carrier frequency energy only in the plate circuit which istransferred by coupling '77 into circuit 78-49 which is tuned to thecarrier frequency.

From the foregoing it is evident that only carrier frequency currentexists in coil '78, and only energy of the side frequencies exists inthe coil '75. Moreover, the phase of the carrier current in coil 78difiers by 90 from the current which would need to be combined withcurrents in coil 75 to constitute the usual modulated wave form.

The voltage due to the side frequency currents in coil '75 supplies thegrid of the radio frequency amplifier 80, and the amplified signals areimpressed through coupling 82 upon the antenna circuit 84. Similarly,the voltage due to the carrier frequency current in coil 78 is impressedupon the grid of the amplifier tube 81, and the amplified signals areimpressed upon the antenna circuit through coupling 83.

In consequence, the antenna current of 84 has the same components and isthe same as would have been produced by the usualtype modulation, exceptthat the phase of the carrier has been changed by 90. The effect of thisalteration of carrier is precisely the same as if the phase of one ofthe side bands of the ordinarily modulated signal had been altered by180, or completely reversed.

As indicated above, this type of transmission has the especial propertyof producing little effect upon ordinary type receivers, because thedetected currents produced by the carrier and one side band areneutralized by the detected currents produced by the carrier and theother side band. The noise produced by beating between side bandsthemselves is small and of poor quality, and often unintelligible.

' With the proper type'of receiver, however, signals may be received ofequally good quality, at equally great distances, as compared with usualtype transmission. Also, the reception of such a signal is subject tomuch less interference than he reception of the usual type receivers.

A receiver for reception of signals produced by the transmitter of-Fig.8 is shown in Fig. 9. This receiver is of the superheterodyne type,employing two detectors in the intermediate frequency. This circuit ispreferred for audio modulated signals, since it permits thereconstruction of the audio wave by circuits of a comparatively lowfrequency and by fixed filter systems, rather than by variable highfrquency circuits as shown in the receiver of Fig. 7. It is to beunderstood that this receiver will receive the signals trans mitted bythe transmitter of Fig. 6.

Referring now more particularly to Fig. 9, as in the usual type ofsuperheterodyne, the incoming signals are picked up and tuned in bycircuit 86 and combined on the input side of the first detector 88 witha local heterodyne frequency supplied by the oscillator 8'7. As aresult, in the plate circuit of the first detector there appears anintermediate frequency determined by the difference between the incomingcarrier frequency and the local frequency. This intermediate frequency,of say 30 kc. is modulated with the side bands in the same relation asthey exist in the incoming signal, so that the result of first detectionis merely to reduce the frequency of the modulated signals withoutchanging their general nature. The energy of this reduced frequencymodulated wave is segregated out by means of filters 89 and 90. suchthat half the energy of the intermediate carrier and the upper side bandis diverted to tube 91, while half the energy of the intermediatecarrier and the lower side band is diverted to tube 92. Thevoltages'impressed on grid circuits of tubes 91 and 92 are different inphase of amplitude variation by 180 degrees. Consequently, on detectionby these tubes, the audio current in the plate circuit of tube 91 is outof phase with respect to the audio current in the plate circuit of tube92. To use both detected currents, the audio energy of both tubes istransferred to a common circuit, say the grid circuit of tube 95, by 120means of transformers 93 and 94, with the relation between primary andsecondary windings in an opposite sense for 94 than for 93. As a result,the effects of the audio currents become additive, and the correctsignal is produced in the telephone receiver 96 in the plate circuit ofthe tube.

It is to be noted, however, that all currents or pulses in the platecircuit of tube 88 which pass through the filters 89 and 90 in equalmanner, will produce effects in phase in the plate circuit of tubes 91and 92, and therefore will produce opposite and neutralizing effects inthe tube 95. For this reason, this system of transmission and receptionis. much more selective against 135 general interference than a,receiver system similar to Fig. 9 for ordinary telephony, but withtransformers 93 and 94 similarly wound instead of oppositely wound.

-I-Iaving thus described several embodiments 140 of my invention, it isto be understood that I am not to be restricted to the forms herein setforth for the purpose of illustration, but by the scope of my inventionas determined by the appended claims.

Having now described my invention, what I claim is:

1. The method of radio transmission and reception which comprisesproducing and transmitting a complex wave form compounded of a 150carrier and two side bands, sending a signal by varying the phase of oneof the three constituents, from that which is necessary in conjunctionwith the others to produce a simple modulated wave form, receiving andindependently detecting the two side bands and combining the resultantfrequencies in the proper phase relationship to reproduce the originalmodulating wave.

2. The method of radio transmission and reception which includesproducing a carrier frequency, producing an audio frequency to betransmitted, modulating said carrier frequency by said audio frequency,sending a message by altering the phase of one of the three componentsthus produced to cause the two side bands to be in opposition, radiatingthe three constituent frequencies thus altered, receiving andindependently detecting the two side bands and combining the resultantfrequencies in the proper phase relationship to reproduce the originalmodulating wave. 1

3. The method of radio transmission and reception which includesproducing a carrier frequency, modulating a portion of said generatedcarrier frequency by an audio frequency to be transmitted, suppressingthe carrier frequency from the resultant of said modulation,transmitting the side bands produced by said modulation, independentlyaltering the phase of another portion of said carrier frequency asgenerated and transmitting it so that the transmitted side bands takenin conjunction with the transmitted carrier frequency are in opposition,receiving and independently detecting the two side bands and combiningthe resultant frequencies in the proper phase relationship to reproducethe original modulating Wave.

4. Apparatus for the transmission and reception of radiant energy whichcomprises means 3 for producing a carrier frequency, means for producingan audio frequency, push-pull means for modulating said carrierfrequency by said audio frequency and eliminating from the outputthereof said carrier frequency, means for radiating the side bandsproduced by said modulation, means for altering the phase of the carrierfrequency 90 electrical degrees, means for radiating the carrierfrequency, the phase of which has been thus altered, means for receivingthe said transmissions including a heterodyne means for detecting theincoming energy, filter means for separating the side components causedby said first detection, means for independently detecting each of saidside components and means for combining the detected audio frequency inthe proper phase relationship.

5. Means for the transmission and reception of radiant energy whichcomprises means for modulating a carrier frequencyby an audio frequency,and signaling means for altering one of the three constituents producedby said modulation from that which is necessary in conjunction withothers to produce a simple modulated wave, means for transmitting thethree frequencies thus produced, means for independently detecting partof the carrier frequency energy and reach of the side band frequencyenergies, and means for combining the energy thus detected in the properphase relationship to reproduce the modulating frequency.

6. Means for the transmission and reception of radiant energy whichcomprises means for modulating a carrier frequency, signaling means forv said transmitter.

quencies, and means for combining the resultant audio frequencies in theproper phase relationship.

7. Means for the transmission and reception of radiant energywhich\comprises signaling means for altering one of the threeconstituents of a modulated carrier frequency transmission from thatwhich is necessary in conjunction with the others to produce a simplemodulated wave form to send a message and means for receiving thetransmission thus produced comprising means for diverting part of thecarrier energy and one side bandto one detector and part of the carrierfrequency and the other side band to the other detector, means forcombining the output energy of the two detectors in such a way that thedetected energy due tothe signal will be additive but detected energiesdue to the stray disturbances will be subtractive.

8. The steps in a method of signalling which comprise producing acarrier frequency wave,

producing a modulating wave, combining a portion of the carrierfrequency wave and the modulating wave and producing from saidcombination two sidebands, transmitting the remaining portion of thecarrier wave and the two sidebands, one of the transmissions having itsphase altered from its normal phase relation with respect to the others,receiving the radiated energy, combining the received energy withlocally generated energy and producing from said combination anintermediate frequency, separating the intermediate frequency energyinto its various constituent parts as determined by said sidebands anddetecting and combining said constituent parts of said energy in theproper phase relationship to reproduce the original signal energy.

9. The steps in the method of radio transmission and reception providingsubstantial freedom from atmospheric disturbances which comprise,producing and transmitting a complex wave form derived from modulating acarrier wave and producing from said modulation side bands, sending asignal by varying the phase relationship of the side bands, receivingand independently detecting the side bands and combining the resultantfrequencies in the proper phase relationship to reproduce the signals. 7

10. A telegraphic signalling system comprising, means for producing acarrier and two related side bands, means including a key circuit forchanging the phase ofsat least one side band with respect to the otherfor sending code messages in accordance with said changes, means forreceiving said radiations comprising a split channel receiver, eachchannel thereof including detecting means, one of said detecting meansbeing operated by the carrier and one side band and v the otherdetecting means being operated by the carrier and the other side band, acombining circuit connected to the outputs of both channels in such-away that the current flowing in said combining circuit varies inaccordance with said changes in phase caused by said key circuit at JOHN HAYS HAMIWOND. JR.

